Efficient conversion of lignin waste and self-assembly synthesis of C@MnCo₂O₄ for asymmetric supercapacitors with high energy density

Lignin waste from the papermaking and biorefineries industry is a significantly promising renewable resource to prepare advanced carbon materials for diverse applications, such as the electrodes of supercapacitors; however, the improvement of their energy density remains a challenge. Here, we design a green and universal approach to prepare the composite electrode material, which is composed of lignin-phenol-formaldehyde resins derived hierarchical porous carbon (LR-HPC) as conductive skeletons and the self-assembly manganese cobaltite (MnCo₂O₄) nanocrystals as active sites. The synthesized C@MnCo₂O₄ composite has an abundant porous structure and superior electronic conductivity, allowing for more charge/electron mass transfer channels and active sites for the redox reactions. The composite shows excellent electrochemical performance, such as the maximum specific capacitance of ~726 mF cm⁻² at 0.5 mV s⁻¹, due to the significantly enhanced interactive interface between LR-HPC and MnCo₂O₄ crystals. The assembled all-solid-state asymmetric supercapacitor, with the LR-HPC and C@MnCo₂O₄ as cathode and anode, respectively, exhibits the highest volumetric energy density of 0.68 mWh cm⁻³ at a power density of 8.2 mW cm⁻³. Moreover, this device shows a high capacity retention ratio of ~87.6% at 5 mA cm⁻² after 5000 cycles.